The present application relates to a method for forming an underlayer composed of a GaN-based compound semiconductor, a GaN-based semiconductor light-emitting element, and a method for manufacturing the same.
In recent years, in order to form a GaN-based compound semiconductor layer having low crystal defect density, various studies have been conducted on methods to grow a GaN-based compound semiconductor layer on a sapphire substrate in a lateral direction (hereinafter, the methods will be referred to as epitaxial growth in a lateral direction or epitaxial lateral overgrowth (ELO)). In general, if plural seed layers that are composed of a GaN-based compound semiconductor and separate from each other are formed on a sapphire substrate, a GaN-based compound semiconductor layer is laterally grown from the seed layers. In the region in which the GaN-based compound semiconductor layer is laterally grown, dislocations propagate only in a lateral direction along with the crystal growth of the GaN-based compound semiconductor layer, but do not penetrate the GaN-based compound semiconductor layer in the vertical direction (thickness direction) of the layer. Therefore, a GaN-based compound semiconductor layer that has low crystal defect density can be obtained.
One of such ELO methods is disclosed in e.g. Japanese Patent Laid-open No. 2002-100579. The technique disclosed in Japanese Patent Laid-open No. 2002-100579 includes a step of forming growth nuclei composed of a nitride semiconductor on a heterogeneous substrate that is not composed of a nitride semiconductor into a cyclic stripe, island, or grid shape, and a growth step of growing nitride semiconductor layers from the growth nuclei so that the nitride semiconductor layers are joined to each other at substantially the midpoints between the growth nuclei and cover the entire face of the substrate. In this growth step, after the nitride semiconductor layers are halfway grown, a protective film is formed above the growth nuclei, and then the nitride semiconductor layers are further grown.
In general, in formation of a GaN-based compound semiconductor layer on a sapphire substrate, the C-plane of the sapphire substrate is used. In a GaN-based compound semiconductor layer formed on the C-plane of a sapphire substrate based on an ELO method, the top surface thereof is the C-plane while the side surface thereof is the A-plane. Specifically, the top surface of the GaN-based compound semiconductor layer is parallel to the {0001} plane of a GaN-based compound semiconductor crystal, and the side surface of the GaN-based compound semiconductor layer is parallel to the following plane of a GaN-based compound semiconductor crystal.    {11 20} plane
Hereinafter, such a crystal plane is expressed as {11-20} plane for the sake of convenience.
Furthermore, the following examples of crystal planes in the hexagonal system are expressed as the {hk-il} plane and {h-kil} plane, respectively, in the present specification for the sake of convenience.    {hkīl} plane    {h kil} plane
In addition, the following examples of directions in the hexagonal system are expressed as the <hk-il> direction and <h-kil> direction, respectively, in the present specification for the sake of convenience.    {hkīl} direction    {h kil} direction
The outline of an existing method for forming an underlayer composed of a GaN-based compound semiconductor and an existing method for manufacturing a GaN-based semiconductor light-emitting element will be described below with reference to FIGS. 9A to 9C and 10A to 10C as schematic partial sectional views of a sapphire substrate and so on.
[Step-10]
Initially, a sapphire substrate 210 of which top surface is the C-plane is loaded into an MOCVD apparatus, and therein the substrate 210 is cleaned in a hydrogen carrier gas for ten minutes at a substrate temperature of 1050° C., followed by decreasing of the substrate temperature to 500° C. Subsequently, based on MOCVD, a trimethylgallium (TMG) gas as a gallium source is supplied, with an ammonia gas as a nitrogen source being supplied, so that a low-temperature GaN buffer layer 211 having a thickness of 30 nm is grown on the surface (C-plane) of the sapphire substrate 210, followed by stop of the supply of the TMG gas. Subsequently, the substrate temperature is increased to 1020° C., and then supply of the TMG gas is restarted, so that a seed layer forming film 212A composed of undoped GaN is grown to a thickness of 2 μm on the buffer layer 211 (see FIG. 9A).
[Step-20]
Thereafter, the sapphire substrate 210 is brought out from the MOCVD apparatus, and strip mask layers 213 composed of nickel (Ni) are formed on the seed layer forming film 212A based on lift-off. The mask layers 213 extend in parallel to the <1-100> direction of the seed layer forming film 212A (see FIG. 9B). After the formation of the mask layers 213, the seed layer forming film 212A is etched by RIE employing a chlorine gas with use of the mask layers 213 as the etching mask, followed by removal of the mask layers 213 (see FIG. 9C). In this manner, strip seed layers 212 extending in the <1-100> direction can be obtained. The width direction of the seed layers 212 corresponds with the <11-20> direction, and the top surfaces of the seed layers 212 composed of undoped GaN are the C-plane. Furthermore, when the width, thickness, and formation pitch of the seed layers 212 are defined as Ws, T, and Wp, respectively, the values of these parameters are as follows: Ws=5 μm, T=2 μm, and Wp=15 μm.
[Step-30]
After the formation of the seed layers 212, the sapphire substrate 210 is loaded into the MOCVD apparatus again, where underlayers 215 composed of GaN are ELO-grown at a substrate temperature of 1050° C. under a pressure of 1×104 Pa. Specifically, the crystal growth of the underlayers 215 composed of GaN is started from the top surfaces and side surfaces of the seed layers 212 (see FIGS. 10A and 10B), so that eventually the spaces between the seed layers 212 are filled with the underlayers 215 to some extent (see FIG. 10C).
[Step-40]
Thereafter, on low dislocation density parts of the underlayers 215, GaN-based semiconductor light-emitting elements such as lasers or light emitting diodes (LEDs) are formed. Specifically, supply of a monosilane (SiH4) gas as a silicon source is started at a substrate temperature of 1020° C. under an atmospheric pressure, so that an n-type first GaN-based compound semiconductor layer composed of Si-doped GaN (GaN: Si) and having a thickness of 3 μm is grown on the underlayers 215. The doping concentration is about 5×1018/cm3.
Thereafter, the supply of the TMG gas and the SiH4 gas is stopped, and the substrate temperature is decreased to 750° C. Subsequently, by employing a triethylgallium (TEG) gas and a trimethylindium (TMI) gas and supplying these gasses through valve switching, an active layer composed of InGaN and GaN and having a multiple quantum well structure is grown.
For example, in order to form a laser of which emission wavelength is 400 nm, a multiple quantum well structure (including e.g. two well layers) composed of InGaN and GaN layers (thicknesses thereof are 2.5 nm and 7.5 nm, respectively) and having an In-content of about 9% is formed. In order to form a blue LED of which emission wavelength is 460 nm±10 nm, a multiple quantum well structure (including e.g. fifteen well layers) composed of InGaN and GaN layers (thicknesses thereof are 2.5 nm and 7.5 mn, respectively) and having an In-content of about 15% is formed. In order to form a green LED of which emission wavelength is 520 nm±10 nm, a multiple quantum well structure (including e.g. nine well layers) composed of InGaN and GaN layers (thicknesses thereof are 2.5 nm and 15 run, respectively) and having an In-content of about 23% is formed.
After the formation of the active layer has been completed, the supply of the TEG gas and TMI gas is stopped, and the carrier gas is switched from nitrogen to hydrogen. Furthermore, the substrate temperature is increased to 850° C., and supply of the TMG gas and a bis(cyclopentadienyl)magnesium (Cp2Mg) gas is started, so that a second GaN-based compound semiconductor layer composed of Mg-doped GaN (GaN:Mg) is grown to a thickness of 100 nm on the active layer. The doping concentration is about 5×1019/cm3. Thereafter, a contact layer composed of InGaN is grown. Subsequently, the supply of the TMG gas and Cp2Mg gas is stopped and the substrate temperature is decreased to a room temperature to thereby complete the crystal growth.
[Step-50]
After the crystal growth has been thus completed, the sapphire substrate 210 is subjected to annealing treatment in a nitrogen gas atmosphere at about 800° C. for ten minutes, to thereby activate the p-type impurity (p-type dopant).
[Step-60]
Thereafter, for example, in a similar manner to a normal wafer process and dicing step, the substrate is subjected to a photolithography step, etching step, step of forming p-electrodes and n-electrodes by metal evaporation, and step of dicing into chips. Furthermore, resin molding and packaging are carried out, so that various light emitting diodes such as round-type diodes and surface-mount-type diodes can be manufactured.
In the above-described existing method for forming an underlayer composed of a GaN-based compound semiconductor and method for manufacturing a GaN-based semiconductor light-emitting element, when the underlayers 215 are ELO-grown in [Step-30], gaps are generated between the underlayers 215 and the surface of the sapphire substrate 210 (see FIG. 10C) because it is difficult to grow the underlayers 215 on the sapphire substrate 210 of which surface is the C-plane.
This results in a problem that the underlayers 215 are readily removed from the sapphire substrate 210. Furthermore, in the case of a GaN-based semiconductor light-emitting element that has a structure in which light emitted from an active layer is extracted through a sapphire substrate, the following problem is also caused. Specifically, as the path for light emitted from the GaN-based semiconductor light-emitting element, two kinds of light paths problematically exist: the path via the underlayer 215 and the sapphire substrate 210; and the path via the underlayer 215, a gap, and the sapphire substrate 210.